The present invention relates to a diffraction grating unit and a second-harmonic generator employing the diffraction grating unit.
A second-harmonic generator which converts light of a fundamental wave to light having a wavelength half of that of the fundamental wave by using a non-linear element has been proposed as a short-wavelength light source. The conversion efficiency of a prior art quasi-phase matching type of second-harmonic generator having a domain reversal structure is especially high and the optical characteristics of the output light are also excellent. In this prior art second-harmonic generator as disclosed in, for example, European Patent Publication No. 0473441 A2, a light return type of resonator employs a diffraction grating to stably oscillates a laser diode which emits a fundamental wave.
In the prior art second-harmonic generator, since light including a second-harmonic is picked up by the diffraction grating, light of the zero-th order and higher is diffracted by the diffraction grating and the second-harmonic is dispersed in various directions. FIG. 9 shows schematically how diffracted rays are generated when input light 20 of a fundamental wave and a generated second-harmonic (SHG) has been incident upon a diffraction grating 6 having a normal 22. The diffracted light includes a zero-order 21 of the fundamental wave and a zero-order of the generated second harmonic (SHG), a first-order 23 of the SHG, a second-order of the SHG, a first-order 25 of the fundamental wave, a third-order 26 of the SHG, a minus first-order 27 of the SHG and a minus first-order 28 of the fundamental wave.
The SHG has a wavelength equal to half that of the fundamental wave and is, therefore, diffracted in more directions than the fundamental wave. Thus, it is impossible to effectively pick up the light output and accordingly, the quantity of light for actual use is fairly small. Since the diffracted light which is not returned to the SHG element is also generated from the fundamental wave, the quantity of light returned to the SHG element is relatively small. As a result, the optical output of the laser diode is correspondingly small, thereby detracting from the conversion efficiency of the SHG element. When the quantity of light returned to the SHG element is small, the oscillation of the laser diode becomes unstable, so that the wavelength of the laser light varies according to changes in ambient conditions such as temperature, vibrations, etc. and thus, a stable phase matching state cannot be maintained.
Meanwhile, in the conventional optical resonator, since a Littrow type of optical system is employed for returning a first-order of a fundamental wave to the laser diode, a second-order of the SHG is oriented in a direction identical with that of the first-order of the fundamental wave. Therefore, the SHG propagates along an optical path identical with that of the fundamental wave so as to return to the SHG element, thus resulting in noise and producing instability in the output due to undesirable resonation.
Furthermore, the known methods have a drawback in that the intensity of the SHG is not variable.
Meanwhile, in the prior art, a prism has been employed as a resonator in which the fundamental wave and the second-harmonic are separated. However, the prism has poor resolution. Thus, the wavelength conversion efficiency is poor when considered over a wide range of oscillation wavelengths.